This invention relates generally to a process for mixing finely divided solid particles.
More particularly, this invention relates to a process for obtaining homogeneous admixtures of two or more unlike and microscopically divided solid substances.
In a specific embodiment, this invention relates to a process for obtaining homogeneous admixtures of pyrotechnic and explosive compositions and forming consolidated shapes therefrom.
It is notoriously difficult to obtain homogeneous mixtures of finely divided solids by conventional dry mixing techniques. As a general rule, the more finely divided the solids are, the more difficult it is to obtain a uniform mixture. Further, differences in particle size, particle shape and specific gravity increase the difficulties in obtaining uniform mixtures.
Even so, compositions comprising consolidated or bonded admixtures of a great variety of unlike particulate materials find substantial commercial and military use. Examples include structural shapes, friction products such as brake pads and clutch facings, pyrotechnic compositions including military rocket motor igniters, polymer bonded explosives, gun propellants and rocket propellants. These examples may be generally characterized as containing a high loading of solid substances including inorganic fillers, fibers, metals and abrasives, inorganic oxidizing salts such as ammonium nitrate and ammonium perchlorate, organic oxidizing and explosive compounds and the like, all bound or bonded by polymers, elastomers and other matrix materials.
A variety of common industrial processes require the mixing or blending of solids with reactive liquids. It is usually necessary to accomplish the mixing or blending before substantial reaction of the liquid occurs. Typical of these processes are those which blend solid materials, often at high loadings, with a monomer of liquid prepolymer, which later reacts to form a solid matrix.
Compounding products containing high solids loadings is often accomplished by use of mechanical mixers which produce a high shear, kneading type of action. These severe mixing conditions are necessary to evenly disperse the solid particles throughout the prepolymer or monomer and to thoroughly wet each of the particles with the liquid. Mixing times are usually prolonged, ranging from many minutes to several hours. Brittle fibers such a glass, graphite or some metal whiskers can not be incorporated into composites by these mixing techniques as they are broken even to the point of being reduced to dust by the severity of the shearing action during compounding. Consequently, brittle fibers are typically incorporated into structural composites as woven fabrics or as mats made by depositing chopped roving onto a conveyor, spraying a binder to hold the fibers in a random arrangement, and heat setting the binder as the conveyor passes through an oven.
The manufacture of polymer bonded rocket propellants and explosives presents an extreme challenge to the compounding art. Solids loading is extremely high, typically above about 85% and desirably above about 90%. The composites must withstand cycling over a large temperature range without cracking or degrading and must have enough strength and resilience to withstand high vibrational and accelerative loads. Dispersion of the solid materials throughout the polymer matrix must be very uniform in order to ensure an uniform burning rate.
Polymer bonded propellants and explosives are typically prepared batch-wise by adding the solids, mainly oxidizers such as ammonium nitrate or perchlorate, or energetic explosives such as RDX, to a liquid prepolymer in a Banbury or similar intensive kneading type of mixer. In one specific process for producing a polymer bonded explosive, five separate mixing cycles are used with staged addition of components during the cycles. Total mixing time for this particular process is in excess of five hours. As can be appreciated, pot life of the mixture is a critical parameter as it must be cast into the desired shape after mixing. Consequently, the polymerization or cross-linking reactions must be held to a very low rate which, in turn, necessitates extremely long cure times for the cast shapes.
There have been attempts made in the past to compound formulations, especially reactive formulations at cold temperatures. For example, Canadian Patent No. 762,437 discloses the preparation of storage-stable compositions containing one or more components which are spontaneously reactive at room temperature. Epoxy adhesive compositions are typical of those disclosed in the patent. The technique employed involved separately freezing an epoxy resin and an epoxy hardener by immersion in liquid nitrogen. Each component was thereafter pulverized and the resulting powders were blended together in liquid nitrogen and were stored for extended periods of time in liquid nitrogen without loss of reactivity upon warming.
A recently declassified report, DTIC Technical Report No. AD516,625, "Low Shear Mixing", Hercules, Inc., Technical Report AFRPL-TR-71-101, July 1971 (Declassified and published February 1984), summarizes the results of a literature search of low shear mixing processes for preparation of a literature search of low shear mixing processes for preparation of propellant compositions. One of the processes described in the report is called "Solid Phase Mixing". As described in the report, solid phase mixing involves the solidification of liquid ingredients by freezing, blending all ingredients while in a finely divided or granular state, casting the blended ingredients, then thawing and curing in the mold. Ingredient addition, mixing and casting are all performed in a vacuum environment to avoid moisture contamination.
The authors of the report, the Bacchus Works of Hercules, Inc., were the same group as did the original demonstration work on the "Solid Phase Mixing" process. The report concluded that the "Solid Phase Mixing" technique was but partially successful and that the blending of various sized solid particles of different densities and loading them into a mold to obtain an intimate mixture was extremely difficult because of segregation due to differences in particle densities and sizes. It was recommended that further work be directed to the preparation of propellant mixes using an inert liquid diluent to lower viscosity, reduce the mixing shear, minimize sensitivity and provide a moisture barrier. At the conclusion of the mixing operation, the propellant mix was to be frozen and granulated. Diluent would then be removed by a freeze drying process and the resulting diluent-free granules were to be cast into a cold mold under vacuum conditions.
U.S. Pat. No. 4,177,227 to Harvey et. al., describes and claims a process similar to that recommended in the DTIC report. Patentees prepare high solids, high viscosity rocket propellants by blending the ingredients with an inert diluent so that mixing can be accomplished at low shear conditions. The mixture is then frozen and granulated after which the diluent is removed by freeze drying. Diluent-free granules are then loaded into a mold under vacuum conditions to consolidate and cure the composition to produce a high solids loaded solid rocket propellant. As can be appreciated, the required freeze drying step is very energy intensive and is difficult to carry out on a large scale. Methylene chloride is the preferred diluent because it exhibits favorable removal characteristics by the freeze dry process and because it is non-flammable. However, it is now recognized that methylene chloride is also a health hazard if breathed by personnel which necessitates added care and expense in practicing the patented process.